A thermostat that heats and cools

Why hysteresis, not a plain threshold

Switch the heater at exactly the setpoint and it chatters on/off every few seconds around the boundary — relays hate that. A hysteresis band (heat below −0.3 °C, stop above +0.3 °C) makes each cycle long and calm. Auto-cooling is a second, wider band on the other side: fan on at +1.5 °C over, off once the room recovers. The overheat alert sits further out still and fires even in Hold mode, because a stuck relay doesn't care about your automation:

if (!hold) {
  // Heating hysteresis: heat below setpoint-0.3, stop above setpoint+0.3.
  if (roomT < setpoint - 0.3f) heating = true;
  else if (roomT > setpoint + 0.3f) heating = false;
  // Auto-cooling: fan on when the room runs 1.5 C past the setpoint, off on recovery.
  if (roomT > setpoint + 1.5f) { if (!fan && evlog) evlog->print("Overshoot - fan ON"); fan = true; }
  else if (roomT < setpoint + 0.5f) fan = false;
}
// Overheat alert regardless of hold: something is wrong (sun on the sensor, heater stuck).
bool hot = roomT > setpoint + 4.0f;
if (hot && !overheat && evlog) evlog->print("OVERHEAT - check the heater");
overheat = hot;

The dashboard

A live setpoint (number widget), both relay states, the alert LED and the trend chart — the whole control panel is a dozen declarations:

dash.layout("Climate", RICON_THERMOMETER);
dash.gauge("Room temp", &roomT, 0, 40, "C");
dash.gauge("Humidity", &roomH, 0, 100, "%").variant("semi");
dash.chart("Trend", &roomT, "C");            // the history everyone wants

dash.layout("Control", RICON_POWER);
dash.number("Setpoint", &setpoint, 5, 35, 1);
dash.led("Heating", &heating);               // relay 1
dash.led("Cooling fan", &fan);               // relay 2 — auto-cooling
dash.led("Overheat!", &overheat);            // buzzer + red LED
dash.toggle("Hold", &hold);
dash.beginAP("RisalDash-Thermostat", "12345678");

The shipped example (examples/04_Templates/Thermostat) runs on the fake day/night room from RisalFakeEnv, so both sides of the hysteresis fire while you watch: mornings heat, afternoons overshoot and the fan spins up. Swap one line for a real dht.readTemperature() and wire two relays when you're happy with the bands.

Making it a product

Persist the setpoint in NVS so it survives reboots; add MQTT + Home Assistant discovery and the thermostat shows up as a climate entity; or let an AI agent read the room and nudge the setpoint via MCP. The full room version — with motion and gas — is the room monitor build.